Effect Of Cryogenic Treatment On Microstructure And Properties Of AZ31 Magnesium Alloy

With the increasingly severe situation of energy shortage,environmental pollution and global warming,all over the world begin to pay more attention to energy saving,emission reduction and environmental protection,and the demand for lightweight and environmentally friendly materials is becoming more and more urgent.As a green environmental engineering material in the 21st century,magnesium and its alloys have been widely used in the automotive,aerospace and 3C electronic communication fields due to their series of advantages such as light density,high specific strength,excellent damping capability,good electrical conductivity and superior electromagnetic shielding performance.However,magnesium alloys have poor plasticity at room temperature,poor wear resistance and corrosion resistance,which also limits the application and development of magnesium alloys to some extent.Cryogenic treatment?CT?refers to the method of optimizing the structure and properties of materials by placing the workpiece in the environment of-190?²30?for a period of time.In the past century,CT has been widely used in carbon steel,tool steel,die steel,high-speed steel and cemented carbide.It has been found that CT can effectively improve dimensional stability,fatigue,wear resistance and corrosion resistance and prolong the service life of workpiece.However,the application of CT on magnesium alloys started relatively late,so the analysis and research on the CT of magnesium alloy has certain theoretical and practical significance.AZ31 magnesium alloy was used in this paper,and the material was annealed at 200C for 2h+CT for different time as cryogenic process parameters.The effects of CT on AZ31 magnesium alloy were studied from the aspects of microstructure evolution,mechanical properties,wear resistance,corrosion resistance,punch shearing and deep drawing formability.The main conclusions are as follows:1.AZ31 magnesium alloy before and after CT is composed of?-Mg matrix and the second phase?-Mg17Al12,but the number of second phase increases obviously and distributed more evenly in the microstructure after CT.In addition,there are some twins can be observed in the matrix after CT,and the number of twins increases further with the increase of cryogenic time.The XRD pattern showed that the intensity of the main diffraction peak of AZ31 changed after CT,and the grain orientation was deflected toward?0002???1010?.2.The tensile strength of AZ31 magnesium alloy after CT increased slightly,from225.2Mpa before CT to 232.9Mpa after CT for 24h.The yield strength decreased slightly,from 167.5Mpa before CT to 161.8Mpa after CT for 6h.The hardness increased significantly,from 69.2 before CT to 75.7 after CT for 6h,increasing by 9.4%.In addition,fracture morphology analysis showed that the fracture mechanism of AZ31unchanged before and after CT,and all were quasi-cleavage fractures.3.The wear resistance of AZ31 magnesium alloy improved after CT.It has the best wear resistance and the lowest wear rate after CT for 18h,which reduced from34.7×10^-12m³m^-11 before CT to 25.6×10^-12m³m^-1,and reduced by 26.2%.Under the same load?or sliding rate?,the wear mechanism of the samples after CT is unchanged,but the surface wear of the samples is slighter than that of the samples without CT.4.CT has significant effect on improving corrosion resistance of AZ31 magnesium alloy.The corrosion potential of AZ31 after CT shifted positively and the corrosion current density decreased.After CT for 6h,the corrosion rate of the samples is 0.0228,which is about 46.6%lower than that of the samples before CT.In addition,the corrosion morphology showed that surface corrosion area of the samples after CT is smaller and the corrosion depth is also shallower.5.CT can improve the punch shearing and deep drawing properties of AZ31magnesium alloy.Compared with the samples without CT,the ultimate shear strength of samples after CT decreases slightly and the rollover radius increases slightly.The proportion of smooth zones in the section increases obviously,and with the increase of cryogenic time,the burr height decreases gradually.In addition,the deep drawing depth of AZ31 magnesium alloy after CT was significantly increased.